1. Field of the Invention
The present invention relates to a road marking composition and to a road marking product comprising in addition to said composition, glass beads and/or anti-skid materials. The road marking composition has photocatalytic properties which, once applied onto a support, provide the same with special characteristics such as self-cleaning properties and a renewable hyperhydrophilic surface. The present invention also relates to the use of anatase type titanium dioxide in a road marking composition or product with a renewable hyperhydrophilic surface.
2. Discussion of the Related Art
A road marking product, whether it lies in either a horizontal or a vertical plane, because of its function undergoes local climatic conditions such as variations in temperature, exposure to ultraviolet radiation (UV) and to pollutants (rain, organic and mineral dust). As a consequence thereof, said road marking product gradually loses its fundamental properties which are the day visibility characterized by the luminance value (QD), the night visibility characterized by the retroreflection (RL) and the marking adherence characterized by the skid resistance (SRT).
Dirt collection negatively affects the marking product colorimetry thus resulting in a loss of cognitive perception for the driver or the user. Indeed, such dirt collection, when deposited onto the glass bead and especially when accumulated into the voids between the glass beads and/or between the anti-skid materials, is “blocked” and partially covers the surface of the beads. As a result, this leads to a high decrease in marking visibility and in particular to a loss of retroreflection that is detrimental to the driver.
Moreover, dirt collection and abrasive wear typically associated with the road traffic cause the road marking product surface to polish, leading to a very high decrease in the skid resistance coefficient, with serious consequences on the road user safety, whether they are motorcyclists, or in particular pedestrians.
Road marking compositions traditionally comprise a binder, pigments, fillers and additives. The use of titanium dioxide as a pigment in the road paints is known. Titanium dioxide exists in various allotropic forms, amongst which the best known are rutile, anatase and brookite. Anatase and rutile type titanium dioxide have a photocatalytic activity. However, the anatase form is much more active than the rutile form.
The photocatalytic activity may be defined as follows. Titanium dioxide (TiO2) is a semi-conductive oxide known for its photocatalytic properties. Indeed, “electron-hole” pairs are photogenerated when such material is submitted to a radiation with a wavelength shorter than 400 nm, i.e. when submitted to a natural radiation (sun) or to an artificial radiation exposure (lamp).
These “electron-hole” pairs react with oxygen, ambient air moisture and hydroxyl moieties or organic products adsorbed onto the titanium dioxide surface and produce radicals, and especially strongly oxidizing superoxide and hydroxyl radicals. The photocatalysis therefore enables to break down organic molecules on the titanium dioxide surface, through the formation of radicals which will initiate the covalent bound breakage. Crystallized titanium dioxide in the anatase form, because of its photocatalytic activity, catalyzes the organic compound oxidation.
Titanium dioxide photocatalytic properties have been used in various applications. Most of studies about TiO2 photocatalysis deal with the mineralization of exhaust gas and how to treat air or water.
The patent WO 97/07069 also describes a self-cleaning glass produced by depositing an optically transparent and abrasion-resistant coating film based upon a photocatalyst which may be anatase type titanium dioxide. Following an air and light exposure, the film, thanks to its photocatalytic properties, prevents dirt and contaminating particles from adhering to the glass.
The patent FR 2 824 846 discloses a substrate comprising on a face thereof a coating possessing photocatalytic properties comprising crystallized titanium dioxide in the anatase form, in a substantially mineral binder comprising at least one semi-conductive metal oxide. The substrates described in this document are generally speaking any material used in buildings and in particular for glazing applications. Coatings thus enable providing the materials they cover with anti-fouling, fungicidal and bactericidal properties, optionally combined with hydrophilic, anti-fogging and optical properties.
Paints comprising photocatalytic compounds are also known, but such coatings, whether they are developed as exterior or even interior spill response products, generally attempt by any means to avoid the binder photodegradation. Such paints comprise silicone type mineral binders and not binders based on organic products, so as to prevent the binder itself from being photocatalytically degraded.
The patent application FR 2 326 461 discloses road marking compositions which may comprise as a pigment a combination of anatase and rutile type titanium dioxide. However, all the illustrative compositions comprising anatase type titanium dioxide are in a solvent phase and comprise as a binder a glycerophthalic alkyd resin and optionally chlorinated paraffins and rubber. Moreover, this document does not mention any special interesting use of anatase type titanium dioxide. This one is absolutely not used for its photocatalytic properties and thus a fortiori not for making a road marking product having a self-cleaning, hyperhydrophilic and renewable surface.